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Constitutive Activity

Constitutive activity of GPCRs may be of relevance for disease development and progression. Firstly, mutations in GPCR genes may cause disease by inducing constitutive activity. Secondly, constitutive activity of GPCRs may be relevant for infectious disease development, as several viruses contain constitutively active receptors in their genomes (see Chapter 15) 12 . Below, a few examples should illustrate how GPCR mutations and virally encoded GPCRs may affect human pathophysiology (Figure...

Studies with membranes have the advantage ofbeing able to eliminate contaminating agonists that may cause an apparent constitutive GPCR activation through multiple rounds of membrane centrifugation and resuspension (see also Chapter 1). In addition, studies with membranes allow for precise control ofthe concentrations of GTP and ions and also of pH, all of which have an effect on constitutive GPCR activity (see Chapter 9) 21-25 . Because ofthe striking sensitivity ofthe apparent constitutive...

Recent studies with the b2AR 67-69 have challenged the existence of a preformed binding site for the agonist (i.e., the lock-and-key scheme where the agonist is simultaneously coordinated by the residues involved in binding the ligand). Instead, it has been suggested that the binding of the agonist and subsequent receptor activation follow a sequential process 4, 68-70 . The sequential binding model operates with several intermediate conformational states between the inactive and the fully...

Several examples of endogenously expressed inverse agonists have been reported. The best characterized endogenous inverse agonist is retinal, which is bound to the light sensor rhodopsin 24 . Rhodopsin has evolved a unique mechanism to minimize basal receptor activity. The chromophore 11-cis-retinal, which acts as an inverse agonist in rhodopsin, is covalently bound to the receptor to ensure extremely low receptor signaling in the dark. Thus, 11-cis-retinal is responsible for the fact that the...

Good starting points for analysis of constitutive GPCR activity by radioligand binding studies are the availability of i an affordable and stable high-affinity tritiated radioligand, and ii various GPCR isoforms with different constitutive activities. The radioligand should preferably be a neutral antagonist, since the binding of neutral antagonists per se is not changed by G proteins 23, 32 , allowing for the analysis of the impact of G proteins on the binding of both inverse agonists and...